Conventionally, in a spark ignition type internal combustion engine, by generating an ion current in a combustion chamber using a spark plug immediately after start of combustion and measuring a generation period of the ion current, the combustion state is determined. For example, Patent document 1 describes a method for measuring characteristics of the ion current from a total time of periods each during which the ion current is larger than a predetermined value or an ion current generation period of the period from ignition to a final point at which the ion current is larger than the predetermined value to detect the combustion state.
Patent document 2 describes a method for setting an ion current determination timing on the basis of a position where the ion current disappears in normal combustion and measuring combustion time on the basis of ion current detection time up to the timing.
Patent document 1: Unexamined Patent Publication No. 6-34491
Patent document 2: Unexamined Patent Publication No. 10-176595